1. Field of the Invention
The present invention relates to an automatic frequency control system, and it especially relates to an automatic frequency control system of a mobile station communicating in a wireless cellular system; the wireless cellular system automatically controls a local frequency signal fL to be substantially synchronized with an input signal SI which is a first received signal SH from a first base station, and it is capable of quickly shifting the frequency of the local frequency signal fL to be close to that of a second received signal Snew from a second base station when the input signal SI is changed from the first received signal SH to the second received signal Snew.
2. Description of the Prior Art
In a wireless cellular system, “synchronization” is one of the major tasks of the receiver. Because of (1) the physical difference among the electronic components (e.g. the frequencies of any two crystal are never exactly the same), (2) the environment effect (e.g. temperature variation will affect the characteristics of the electronic components), and (3) especially the Doppler Effect, etc, the frequency or the carrier frequency of a received signal at the receiver side is usually different from the desired one. Such a case may cause the receiver to fail or perform badly due to the frequency at the receiver side not being well synchronized with that of the received signal.
In order to solve such a problem, all the receivers in conventional wireless cellular systems comprise a device or a software called the automatic frequency controller. The automatic frequency controller can adjust a local frequency signal fL, on which the receiving operation relied on, to approach the frequency of the input signal SI, which is the signal transferred from the base station and received by the receiver. By using the automatic frequency controller, the frequency error fE between the local frequency signal fL and the input signal SI is controlled to be at a level suitable for receiving operation.
In practical use, the automatic frequency controller is of course not a perfect one which means the frequency error fE is unavoidable. Therefore, the receiver design should be able to tolerate the existence of the frequency error fE. To be more specific, some modules, e.g. the equalizer in the receiver, should be able to operate normally under a certain range of frequency error fE. Please note that a requirement of tolerating a wide range of frequency error fE will usually not only increase the difficulty of the hardware design but also downgrade the stability of the equalizer. This is definitely not desirable to the system designer since the cost will go higher while the performance of the product will go down.
Referring to FIG. 1, FIG. 1 is a schematic diagram of the wireless cellular system 10. In the wireless cellular system 10, a mobile station (MS) 12 moves around a plurality of Base stations (BSs) and chooses a suitable BS as its serving BS H. Because both the location of the MS 12 and the overall environment are always changing, in a practical system like the Global System for Mobile (GSM), the MS 12 will not only periodically monitor the signal from the serving BS H but also monitor the signal from the other BSs, which might be the next possible serving BS for the MS 12. If necessary, the MS 12 will then change its serving BS from the current one to a new one to ensure MS's communication.
Referring to FIG. 1 and FIG. 2, FIG. 2 is a schematic diagram of the automatic frequency controller 18 of the MS 12 shown in FIG. 1. The conventional automatic frequency controller 18 comprises a frequency detection module 19, a processing module 20, and a frequency generator 22. The frequency detection module 19 is used for comparing the input signal SI with the local frequency signal fL and generating the frequency error fE between the input signal SI and the local frequency signal fL. The frequency detection module 19 is well known to those skilled in the art. The processing module 20 is used for receiving the frequency error fE and generating a frequency control signal Vt. The frequency generator 22 is used for generating the local frequency signal fL according to the frequency control signal Vt. Practically, the frequency generator 22 is, as an example, a voltage controlled oscillator (VCO). The local frequency signal fL is then fed back to the frequency detection module 19 to form a feed back control loop. In a normal operation, the automatic frequency controller 18 will make the local frequency signal fL to be synchronized with the input signal SI when the automatic frequency controller 18 reaches its steady state.
However, some problems may arise when the input signal SI is changed from a first received signal SH from the serving BS H to a second received signal Snew from a second BS Hnew (e.g. for the case that the MS 12 needs to change its serving BS), which means that the frequency of input signal SI of the automatic frequency controller 18 will suddenly change to a new one. In such a case, at the very beginning, the frequency error fE in the automatic frequency controller 18 will be increased due to the frequency change of the input signal SI. The processing module 20 will generate the frequency control signal Vt based on the frequency error fE to control the local frequency signal fL to approach the input signal SI. The automatic frequency controller 18 will eventually make the local frequency signal fL to be synchronized with the input signal SI, which is now the second received signal Snew from the second BS Hnew, again. However, the automatic frequency controller 18 will need a certain amount of time to recover its synchronized status when the frequency of the input signal SI is changed. In other words, at the very beginning, when the frequency of the input signal SI is changed, the frequency of the local frequency signal fL is still kept at the original one corresponding to the original serving BS H. In the MS 12, the local frequency signal fL provided by the frequency generator 22 acts as a basic clock signal for the receiving operation. Especially, some modules, e.g. the equalizer, in the receiver of the MS 12 is very sensitive to the frequency error fE between the local frequency signal fL and the input signal SI. However, the equalizer of the MS 12 receiver can usually only tolerate a certain range of the frequency error fE when the MS 12 receiver is communicating with the BS. The situation that the frequency of the input signal SI is quickly changed may cause too large a frequency error fE, such that the MS 12 receiver can not take it, and the system performance is downgraded. Therefore, it is desirable to resolve such a problem.